Minimizing corrosion and build-up in a flue-gas system including a desulfurizer

ABSTRACT

A method for minimizing corrosion and the build-up of deposits on surfaces of a flue-gas system exposed to moist substances and elevated temperatures, and which includes a desulfurizer and heat transfer means communicating with the desulfurizer, said method involving adding to the system at the heat transfer means a readily water soluble alkaline substance such as sodium hydroxide in amounts sufficient to produce with said flue-gas in said heat transfer means a minimum pH of about 5, preferably 7 or higher.

[0001] The present invention minimizes corrosion and build-up in aflue-gas system including a desulfurizer and heat transfer meanscommunicating with the desulfurizer where significant amounts ofmoisture and/or sulfuric acid are present by adding to the flue-gaswhile it is at a relatively high temperature a readily water solublealkaline substance such as sodium hydroxide in an amount sufficient toproduce with said flue-gases as they leave the heat transfer means a pHof a minimum of about 5, preferably 7 or higher.

BACKGROUND OF THE INVENTION

[0002] In most flue-gas systems, for safety and environmental reasons,as a means of conserving heat, the flue-gas leaving the furnace atrelatively high temperatures is passed through a variety of treatmentdevices before escaping into the atmosphere. Among these devices are,usually in sequence, a boiler or heater, a precipitator, a heat transferdevice such as a gas/gas heater, and a scrubber or desulfurizer, theflue-gas then returning to the gas/gas heater on its way to the stack.The temperature of the flue-gas decreases as the gas passes through thesystem, and in the course of that temperature decrease moisture,originally in the form of steam, and even containing sulfuric acid,comes into being as a liquid which deposits on the surfaces of thesystem, and particularly on the gas/gas heater surfaces, the sulfuricacid content of that liquid producing corrosion and the solid content ofthe flue-gas tending to deposit and build-up on exposed system surfaces.

[0003] It has long been customary to add substances to the flue-gas tominimize or prevent corrosion of the exposed surfaces of the system. (Myprior U.S. Pat. Nos. 4,842,617 of Jun. 27, 1989 entitled “CombustionControl By Addition of Magnesium Compounds of Particular ParticleSizes”, and 5,034,114 of Jul. 23, 1991 entitled “Acid NeutralizingComposition Additive With Detergent Builder” are representative of theuse of such additives, as is a pending U.S. application Serial No.814,598 of Mar. 23, 2001 entitled “Use Of Expanded Agents For MinimizingCorrosion And Build-Up Of Deposits In Flue-Gas Systems”, the inventionof myself and William Carmen Pepe.) The corrosive action of sulfuricacid on exposed surfaces of the system is obviously undesirable and itis therefore common to add such substances as limestone or magnesiumoxide to the system to neutralize the sulfuric acid. Because asolid/liquid reaction rate is generally slow, relatively large amountsof such additives must be provided. They are usually pneumaticallyinjected into the affected portion of the system through conduits,usually in the form of pipes, using pressurized air as the vehicle totransport the additives through the conduit to the injection location inthe system. The act of compressing air generates both heat and moisture,and hence the pressurized air which does the conveying is usually bothmoisture-laden and hot. Movement of the pressurized additive through theconduits results in some condensation of the moisture on the conduitsurface and this enhances the tendency of the solid additives to stickto and build-up on those surfaces. As a result it is periodicallynecessary to take the injection equipment off line for cleaning, aprocess which is itself costly and time consuming, and while theinjection equipment is off line no anti-corrosion additive is fed to thesystem, thus increasing the likelihood of corrosion.

[0004] When the system is provided with a scrubber or desulfurizer theflue-gas emanating from the scrubber has a comparatively high moisturecontent and a comparatively low temperature, thus leading to thecondensation of comparatively large volumes of moisture, significantlyincluding sulfuric acid in its liquid form because its temperature isbelow its dew point. When, as is usually the case, the output from thescrubber is fed back to the gas/gas heater the moisture content of theflue-gas becomes a significant corrosion-producing factor.

[0005] Previous attempts to solve this problem, as for example theinjection of lime, limestone, magnesium oxide, magnesium hydroxide, orother basifying agents that are insoluble in water, have two majordrawbacks. Because these products are insoluble in water, they are mostunreactive or very slowly reactive, particularly where theneutralization of the acidity takes place in the steam which existsbecause of the high temperatures involved, and as a result excessamounts of basifying agent relative to the amount of SO₃ that has to beneutralized are required, and the use of such a large excess of theseinefficient basifying agents results in clogging and deposit build-up onthe gas/gas heater inlet plates, rendering the gas/gas heatersinoperative.

SUMMARY OF THE INVENTION

[0006] We have discovered that corrosion of the system surfaces, andparticularly the inlet surfaces of a scrubber and gas/gas heater used inconjunction therewith, can be significantly reduced, and the build-up onthose surfaces of residue particularly from the substances describedabove, which have been added to the system for various purposes, canlikewise be significantly reduced or even substantially eliminated, bynot using such substances at all, but rather by applying to theflue-gas, while it is in the heat transfer means upstream of saidscrubber, where its temperature is well above the boiling point ofwater, an alkaline substance which is readily water-soluble in an amountsufficient to bring the flue-gas-additive combination as it leaves theheat transfer means to a minimum pH 5, preferably 7 or higher. Inparticular, the alkaline material, preferably in the form of a watersolution, is added to the flue-gas while the moisture content of theflue-gas is in the form of steam. Significantly, the amount of suchalkaline substance which must be added is in general that amountrequired to neutralize the acid content of the flue-gas. This in itselfis a significant factor, since the prior art additives, generallyinsoluble or difficultly soluble, must be added in amounts substantiallygreater than the stoichiometric amounts in order to obtain the desiredneutralization but by the same token increasing the build-up which is soundesirable. With the use of this invention the SO₃ acid dew point ofthe vapor stream from the heat transfer means is reduced from above 300°F. to 250° F. or even lower which minimizes condensation of sulfuricacid in the gas/gas heaters, even at the high humidities present at theheater plates. The actual untreated SO₃ dew point is dependent upon theamount of SO₃ present in the flue-gases. A dew point of 300° F.represents a concentration of 61 parts per million of sulfuric acidwhile an acid dew point of 250° F. represents a concentration of only 2parts per million.

[0007] The amount of basifying agent required can be determined bymeasuring the acid dew point of the gases in or leaving the gas/gasheater, which can be done manually, or preferably with a continuous,on-line, automatic acid dew point meter, and the using of the latter toregulate the feed rate of the aqueous basifying agent.

BRIEF DESCRIPTION OF THE DRAWING

[0008]FIG. 1 discloses diagrammatically a typical flue-gas system inwhich the method of the present invention is particularly useful.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] A typical flue-gas system such as is shown in FIG. 1 comprises afurnace or boiler 2 where steam is generated. Ambient air enters thesystem at 4 and passes through a primary air heater 6 in which it isheated to perhaps 150° F. and it then enters the furnace 2 to combinewith fuel for combustion purposes. A waste product from the combustionin the furnace 2 is the flue-gas which exits the furnace at 8 at atemperature of perhaps 800° F. The flue-gas passes through the airheater 6, providing the means for the initial heating of the ambientair, and the flue-gas which leaves the air heater 6, at 10, will havelost a great deal of its heat and be at a temperature of about 350° F.to 400° F. It then passes into an electrostatic precipitator 12 in whichcertain impurities are removed, and it escapes from the precipitator 12at 14 at a further reduced temperature of about 275° to 300° F. Becauseof its reduced temperature the flue-gas may now have a significantmoisture content of perhaps 5-15%. The flue-gas goes into the upperportion 16A of a heat transfer means, here shown as a gas/gas heater 16,from which it escapes to point 18 at a temperature of less than 250° F.and it then passes through a desulfurizer or scrubber 20 which it leavesat point 22 at a temperature of perhaps 100° F.-150° F. and with amoisture content of perhaps 40-50%. The gas is then fed back through thelower portion 16B of the gas/gas heater 16 and escapes through the stackat 24.

[0010] The gas/gas heater 16 has structural parts which move from theupper hot portion of 16A to the lower or relatively cool portion of 16Band back again. It will be apparent that exposed surfaces of the gas/gasheater 16, and particularly those surfaces thereof which at any givenmoment are in the lower portion 16B of the heater, are very susceptibleto acid corrosion because of the high moisture content and lowtemperatures to which they are subjected, so that the metal temperatureis below the acid dew point. From the point of view of minimizingcorrosion in the gas/gas heater 16 it is at the area 14 immediatelyup-stream of the gas/gas heater 16 where the usual corrosion-minimizingadditives are injected into the system, as indicated by the arrow 26.

[0011] The susceptibility of the gas/gas heater 16 to corrosion canperhaps be best appreciated by considering that scrubber 20 more easilyand effectively absorbs impurities from the flue-gas when the flue-gasis at or below its dew point, and when the flue-gas exits the scrubber20 its temperature is below the dew point to an even greater degree,thereby increasing its moisture content and making corrosion morelikely. Also, because structural parts of the gas/gas heater 16 movesequentially through the upper and lower portions 16A and 16B thereof,they are constantly subjected to variations in temperature, and theconstant heating and cooling of the structural parts of the gas/gasheater 16, coupled with the resultant high moisture content of theflue-gas as that passes through the heater, produces a situation idealfor corrosion and for deposit build-up.

[0012] Also, with the previous usage of lime, limestone or magnesium,additive builds up in the conduit feeding those additives to the system.The additives are preferably injected into the system between theprecipitator 12 and the gas/gas heater 16, as indicated by the arrow 26,so that they can perform their desired action where that action is mostneeded, to wit, in the gas/gas heater 16.

[0013] The conventional, previously used, anti-corrosion additives, suchas calcium oxide, calcium hydroxide, calcium carbonate, dolomite,dolomitic lime, lime, calcium hydrate, limestone, magnesium oxide,magnesium hydroxide, magnesium carbonate, as well as combinationsthereof such as calcium/magnesium oxides and hydroxides, are relativelyineffective because of the relative slowness of the reaction betweenthese basifying additives and the sulfur trioxide that they are designedto neutralize, and those additives must be provided in relatively largequantities, well in excess of the stoichiometric amount required toneutralize the acidic constituents. As a result the problem involved inpreventing build-up in the conduits through which those basifying agentsare fed is intensified, and the inevitable deposit build-up on thegas/gas heaters results in a rapid shut down of the scrubber.

[0014] According to the present invention the build-up problem, both inthe additive conduit and on the perforated gas/gas heater revolvingplates, is eliminated and the corrosion problem, particularly in thegas/gas heater 16, is minimized or virtually eliminated with the use ofthe water-soluble basifying agents described herein added to the systemas shown.

[0015] In the embodiment of the present invention here specificallydisclosed the system comprises a desulfurizer or scrubber 20 functioningin combination with a heat transfer means here specifically shown as agas/gas heater 16. The upper portion 16A of the gas/gas heater 16 is ata higher temperature than its lower portion 16B. In particular, thetemperature in the upper portion 16A is well above the boiling point ofwater. Hence the water content of the flue-gas is in the form of steam.It is when the flue-gas enters the lower portion 16B of the gas/gasheater 16 that, because of the lower temperature in the region, thesulfuric acid content of the flue-gas is at or below its dew point, as aresult of which corrosive sulfuric acid, if present, tends to form onthe exposed surfaces of the gas/gas heater 16.

[0016] In accordance with the present invention, we take advantage ofthe high temperature of the flue-gas at the upper portion 16A of thegas/gas heater 16 to subject the flue-gas at that relatively hightemperature to the action of a readily water soluble alkaline substance,preferably in the form of a water solution. Under those conditions thealkaline substance reacts substantially immediately and completely withthe acid content of the flue-gas, neutralizing that acid content sothat, when the flue-gas exits from the scrubber 20 and returns to thelower section 16B of the gas/gas heater 16 in a condition below its dewpoint, there will be little or no acid content available to producecorrosion. Thus by adding the alkaline substance in an area where,because of the high temperature, the liquid content from the flue-gas isin the form of vapor, the sought-for neutralization action iseffectively achieved. Best results are obtained if the pH of theflue-gas after it leaves gas/gas heater 16 is at a minimum of 5, andpreferably 7 or greater, and the optimum amount of alkaline substanceadded is that which produces the specified pH at that point in thesystem. This can, of course, be readily monitored by available sensorsand control equipment.

[0017] Preferred alkaline substances for use in connection with thepresent invention are sodium hydroxide and sodium carbonate primarilybecause they are low cost materials, and the fact that the corrosion andbuild-up inhibition can be achieved with stoichiometric amounts of suchlow cost material is an exceedingly important advantage of the presentinvention. However, other substances having the same characteristics ofalkalinity and ready water solubility could be employed. When certainchemicals often used in water treating systems are co-added with sodiumhydroxide or sodium carbonate, this enhanced corrosion protection, andparticularly helped to prevent clogging of the gas/gas heater openingseven beyond that which was achieved with the use of either sodiumhydroxide or sodium carbonate by itself. Particularly effective asco-additives were trisodium polyphosphate, trisodium phosphate, disodiummonohydrogen phosphate, sodium borate, sodium di and polyborates, sodiumsilicates and sodium polysilicates, and in general water soluble sodiumsalts of the various phosphates, silicates and borates.

[0018] The effectiveness of the use of these basifying products having ahigh initial pH in minimizing corrosion or deposit build-up is shown bythe following laboratory demonstration. In each of the following samplesa mixture of 30 cc of water, 3 cc of 6 Normal sulfuric acid and 2 cc ofthe additive as described in Table I was observed after incubation at130° C. for one hour with the results as set forth in Table I. TABLE IBasifying Agents to Prevent Corrosion of Gas/Gas Heaters Connected to aDesulfurizer Results After Incubating at 130° for 1.5 Hours with SteelMetal Strip Exposed to Condensate from Sample Composition the Mixture ofDilute No. of Additive Sulfuric Acid and Additive 1 No additive Heavycorrosion, ⅛″ of a de- posited, brown, layer on bottom of metalspecimen. 2 14 cc of 1.2 N No corrosion, very slight, caustic solutionscattered, brownish spots; trace of brown deposits on bot- tom ⅛″ ofmetal specimen. 3 18 cc of 1.2 N No corrosion; deposit-free met- causticsolution al specimen, trace spotting on bottom ⅛″ of metal specimen.

[0019] Without the basifying additive, rapid corrosion of the steelmetal strips takes place. In each case, the additive increased the pH ofthe solution from below 3 to over 8.

[0020] In another series of experiments as shown in Table II, use ofcombining basifying chemicals with deposit modifying, or anti-corrosion,enhancing chemicals show a further improvement when this combination isused. TABLE II Use of Combined Basifying Chemicals and Deposit-Modifyingor Anti-Corrosion, or Enhancing Chemicals Are Used In CombinationResults After Incubating at 130° for 1.5 Hours with Steel Metal StripExposed to Condensate from Sample Composition the Mixture of Dilute No.of Additive Sulfuric Acid and Additive 1A None Metal completely coveredwith brown, rust-like, stain. Very heavy brownish/black coating onbottom ¼″ of metal specimen. 1B 18 cc of 1.2 N Essentially clear, veryslight, caustic solution scattered, brown spots, trace deposit on bottom⅛″ of metal specimen. 1C 15 cc of 1.2 N Totally clear, deposit-free,caustic solution, metal strip, no trace of de- plus 0.70 cc of a positon bottom ⅛″ of metal 3% solution of specimen. trisodium phosphate 1D 15cc of 1.2 N Totally clear, deposit-free, caustic solution, metal stripthroughout. plus 0.70 cc of a 3% solution of sodium tripolyphosphate

[0021] The total amount of additives required is based on the flow ratesof the flue-gas itself and the recirculating water solution from thescrubber 20, as well as the acidity present in the system. Basically,the feed rate of additive is determined primarily by the acidity of thestream and that amount of basifying agent, or basifying agent withmodifier, that decrease the acid dew point of the stream as it leavesthe gas/gas heater 16 from plus 300° F. to 250° F. or less.

[0022] With a boiler of 200 megawatts, an SO₂ content of 6000 mg/Nm, andsulfuric acid content at the gas/gas heater of 30 ppm (112 mg/m³) andwith a treatment rate of 600 ppm of a 5% solution of caustic, thefollowing results were obtained. The acidity was reduced to 5.0 mg/Nm,or less than 2 ppm or 7.5 mg/m³. With the additive combination as shownin Example 1C of Table II, a treatment rate of 600 ppm reduced theacidity to 2.1 parts per million, or 7.8 mg/m³ without any corrosion ortraces of deposits on the gas/gas heater plates.

[0023] The most cost effective treatment rates may vary from boiler toboiler and will depend upon the megawatts of the boiler, the temperatureat the inlet and outlet of the gas/gas heater, the acidity of the returnflow rate from the scrubber to the gas/gas heater, the design of thegas/gas heater and the amount of sulfur dioxide and sulfuric acidpresent and the amount of moisture in the condensate. Measurement of theacid dew point of the stream immediately prior to or after the gas/gasheater may be made by employing an on-line automatic acid dew point testapparatus, such as a Land Continuous Dewpoint Acid Monitor, which inturn can be connected to an automatic feed adjusting system thatmonitors the inlet feed rate of the aqueous basifying agents. Thecontrolling feed rate is that amount of additive that increases the aciddew point of the circulating acid solution from the scrubber to below250° F.

[0024] While but a single preferred embodiment is here specificallydisclosed, it will be apparent that many variations may be made in thedetails of the method here disclosed, all within the scope of theinstant invention as defined in the following claims.

We claim:
 1. In a flue-gas system in which the flue gases, at atemperature above the boiling point of water, pass through a heattransfer means on their way to a desulfurizer, a method of minimizingcorrosion and build-up by applying to said flue gases at the said heattransfer means a readily water-soluble alkaline substance in an amountsufficient to produce with said flue gases a pH of a minimum of
 5. 2. Ina flue-gas system in which the flue gases, at a temperature above theboiling point of water, pass through the hot side of a heat transfermeans having a cold side and a hot side on their way to a desulfurizerand then pass from the desulfurizer through the cold side of said heattransfer means, a method of minimizing corrosion and build-up byapplying to said flue gases at the said hot side of said heat transfermeans a readily water-soluble alkaline substance in an amount sufficientto produce with said flue gases a pH of a minimum of
 5. 3. In a flue-gassystem in which the flue gases, at a temperature above the boiling pointof water, pass through the hot side of a heat transfer means having acold side and a hot side on their way to a desulfurizer and then passfrom the desulfurizer through the cold side of said heat transfer means,a method of minimizing corrosion and build-up by applying to said fluegases at the said hot side of said heat transfer means a readilywater-soluble alkaline substance in water solution in an amountsufficient to produce with said flue gases a pH of a minimum of
 5. 4.The method of any of claims 1-3, in which said alkaline substance is amember of the group consisting of sodium hydroxide and sodium carbonate.5. The method of any of claims 1-3, in which said application to saidflue gases includes, in addition to said alkaline substance, one or moreof the following substances: water soluble sodium, potassium, or cesiumphosphates, silicates and borates and other water soluble anionic saltsof said elements having an initial pH of a minimum of
 5. 6. The methodof any of claims 1-3 in which, as the flue-gas system functions, the pHof the combination of flue gas and alkaline substance is monitored andthe rate of addition of said alkaline substance to said flue gas ismodified in accordance with that monitoring in order to substantiallymaintain the desired pH value.